In series manufacture of vacuum brake power boosters it is necessary to adjust the difference between the reaction disc and the second piston part, that is to say, the so-called parameter z, in order to obtain the characteristic curves demanded by the automobile industry. Indeed, the parameter z to be adjusted determines the behavior of the booster in the initial phase of its actuation, in particular the extent of the jump of the output force when being actuated with an established input force (so-called jumper effect).
In an adjustable vacuum brake power booster which is known from the German patent application published without examination, No. 3,641,604, the adjustment of the desired parameter z is carried out when the piston rod is not incorporated, which is pressed into an elastic ring-shaped piston rod holder positioned in the first piston part. In the prior art vacuum brake power boosters the piston rod holder which is made of elastic material cannot transmit any elevated plucking-out forces which are demanded by the automotive vehicle manufacturer.